1. Field of the Invention
The present invention relates to a polymer, a polymer electrolyte membrane for polymer electrolyte fuel cells, and a membrane/electrode assembly.
2. Discussion of Background
A polymer electrolyte fuel cell is, for example, a stack of a plurality of cells each comprising a membrane/electrode assembly sandwiched between two separators. The membrane/electrode assembly comprises an anode and a cathode each having a catalyst layer and a polymer electrolyte membrane disposed between the anode and the cathode.
As a polymer to be used for the polymer electrolyte membrane, the following polymer has been widely used.
(1) A polymer obtained by subjecting a copolymer of a monomer represented by the following formula and tetrafluoroethylene to conversion to an acid form to convert —SO2F groups to sulfonic acid groups (Non-Patent Document 1):CF2═CF—(OCF2CF(CF3))m—O—(CF2)n—SO2Fwherein m is an integer of from 0 to 1, and n is an integer of from 1 to 6.
For a polymer electrolyte fuel cell, further improvement in power generation performance is required, and therefore, as a polymer to be used for a polymer electrolyte membrane, a polymer having a low electrical resistance i.e. a low equivalent weight has been desired. In order to reduce the equivalent weight of the polymer (1), the number of sulfonic acid groups should be increased, that is, the proportion of the monomer represented by the above formula should be increased.
However, if the proportion of the monomer represented by the above formula is increased, a sufficiently high molecular weight of the polymer will hardly be achieved, and further, the polymer will be excessively swollen with water, whereby the mechanical strength of the resulting polymer electrolyte membrane will be insufficient. Further, in an operating environment of a polymer electrolyte fuel cell, the polymer electrolyte membrane undergoes repeats of swelling in a wet state and shrinkage in a dry state, whereby the polymer electrolyte membrane is likely to be cracked and damaged. As a result, the durability of a membrane/electrode assembly will be insufficient.
Non-Patent Document 1: W. Vielstich, H. A. Gasteiger, A. Lamm, “Handbook of Fuel Cells, vol. 3”, U.S. John Wiley & Sons, Ltd., 2003, Chapter 30, p. 351-352